docs: def missing_labels(self):
          """
          A 1D `~numpy.ndarray` of the sorted non-zero labels that are
          missing in the consecutive sequence from zero to the maximum
          label number.
          """
  
          return np.array(sorted(set(range(0, self.max_label + 1)).
                                 difference(np.insert(self.labels, 0, 0))))